The invention relates to a mounting for the blowing mandrels of a blow-molding device for producing hollow bodies of thermoplastic.
The usual containers of tin plate or ferrous metal, glass, or even ceramic of the past are increasingly being replaced by plastic containers. Especially for packaging fluid substances, such as beverages, oil, cleaning substances, cosmetics, and so forth, plastic containers are primarily employed. Surely the low weight and lesser costs play a not inconsiderable role in this replacement. The use of recyclable plastic materials and the overall more-favorable total energy balance to produce them also contribute to promoting consumer acceptance of plastic containers and especially plastic bottles.
The production of plastic containers, especially plastic bottles, is typically done by an stretch blow molding process or an extrusion blow molding process. In the stretch blow molding process, a predominantly cylindrical parison, open on one end and produced separately by injection molding, is placed in a blow mold. After that, a blowing mandrel is introduced through the opening into the parison. The heated parison is reshaped by stretching and inflation with overpressure to fit the mold cavity. In the extrusion blow molding process, a single- or multi-layer extruded parison, such as a hose, is placed in a blow mold and inflated by overpressure via a blowing mandrel to fit the mold cavity. Depending on whether the hoselike parison is extruded continuously or is temporarily stored upstream of the tip of the worm or in an extrusion head especially embodied for the purpose and ejected spontaneously, “in batches”, into the blow mold cavity, the method is called either a continuous or a discontinuous extrusion blow molding process.
The blow-molding devices employed for this often have multiple-mold tools with a plurality of blow-molding cavities, which make it possible to produce a plurality of plastic containers simultaneously in one operation. Each blow-molding cavity is assigned at least one separate blowing mandrel, which is positionable appropriately for the process.
Hollow plastic bodies such as bottles or canisters, but also industrial parts, in many cases have one or more openings that perform a particular purpose in their later use. Often, such openings are embodied such that they can be closed by suitable closing devices. These may for instance be screw closures or snap closures. An essential demand made of the closing technology is tightness to liquids, gases and solids in powdered or granular form. In plastic packages, the tightness is typically accomplished by creating smooth, dimensionally accurate sealing faces, adapted to one another, in both the hollow body and the closure. It is known that the sealing quality can be improved still further by the choice of the correct combination of materials for the hollow body and the closure.
The production of openings designed in this way can be done simultaneously with the production process of the hollow body, or by mechanical postmachining of the hollow body. The opening for introducing the inflation medium, in most cases compressed air, that generates the overpressure is designed at the outset such that it also meets the demands later made in terms of use of the hollow body. The sealing faces, which may be provided both on the end face and on a cylindrical or conical inner wall of a neck that surrounds the opening, are produced by a so-called calibration process, that is, a dimensional adaptation of a portion of the blowing mandrel, acting as a calibrating device, to the region of the opening in the blow-molding tools and to the predetermined wall thickness of the parison inserted into the blow-molding tools. The blowing and calibration mandrel furthermore has the task of pinching off process-created protruding parts of the parison in the upper peripheral region of the opening, except for a minimal residual thickness of the hollow body, so that they can be stamped out in a deburring process that follows the inflation operation.
For both operations, that is, calibrating the opening and pinching off protruding parts, the blowing and calibrating mandrel should assume a very accurate, replicable position relative to the opening in the blow-molding tool in three planes. In particular, the axis of the blowing and calibrating mandrel should match the axis of the opening to the cavity in the blow-molding tool; the intersecting annular face of the blowing and calibrating mandrel should be parallel to the neck knife face of the blow-molding tool, and the final position in terms of height of the blowing and calibrating mandrel relative to the opening in the blow-molding tool should be at exactly the position at which both the dimensional tolerances of the finished hollow body are maintained, and the pinching off of the protruding parts is assured. In practice, this is attained by setting a mechanical end stop for the blowing and calibrating mandrel. This setting is often done manually, by visual assessment of the pinching operation and dimensional monitoring of the opening of the hollow body on the part of the machine operator. In a single blow molding system that has only one blowing and calibrating mandrel, the two demands in terms of quality with respect to the opening and to the protruding parts can still be met simultaneously relatively simply. In automatic blow molding systems with a plurality of parallel blow-molding tools and blowing and calibrating mandrels assigned to them, however, the setting can be done only with great difficulty and is very time-consuming with a view to the demands of quality in the pinching operation, the individual blowing and calibrating mandrels should all be set to the correct height relative to the openings of the cavities in the blow-molding tools. In this respect it should be taken into account that differences in dimension at the openings of the individual blow-molding tools, which are due to the usual production variations or to wear in any case, make different height adjustments of the blowing and calibrating mandrels necessary. Because of the risk of injury to the machine operator, the components of these multiple automatic blow molding systems that cooperate mechanically with one another are protected by covering devices against unauthorized or unintentional access. However, these structural protective provisions are a hindrance to a practicable capability of setting, which should advantageously be done with the machine running.
From International Patent Disclosure WO 01/62472, it is known for the blowing and calibrating mandrels to be mounted axially freely movably within predeterminable limits. In the operation of jointly positioning the blowing and calibrating mandrels against the blow-molding cavities, the blowing and calibrating mandrels are automatically adjustable in height, within the predeterminable limits relative to their outset position, counter to the resistance of a coupling medium, such as a hydraulic fluid. The result is an automatic self-setting of the blowing mandrels which assures a correct position of the blowing mandrels in terms of height.
For technical reasons involving assembly and for maintenance, the blowing and calibrating mandrels are mounted detachably in the blow-molding device. In a modern system, of the kind described for instance in WO 01/62472, they are fixed in their mountings by mechanical clamping. The clamping force is selected such that the blowing and calibrating mandrels cannot adjust unintentionally as a result of their weight and the dynamic forces involved in the processes of motion. The blowing and calibrating mandrels are prevented from falling out of the blow molding system by means of stops. The mechanical clamping mounting includes a retaining nut with a male thread that can be screwed into a threaded bore in the upper machine part. The retaining nut has a conical face that cooperates with a radially compressible clamping element that has a conical counterpart face. The cooperating conical faces divert the force that results from the tightening moment of the retaining nut and, by the radial narrowing of the clamping element, assure the requisite clamping force. A prestressing element, such as a cup spring, compensates for dimensional tolerances and nonuniformities in the formation of the thread, the threaded bore in the upper machine part, and the retaining nut.
The mechanical clamping mounting offers the capability of warp-free mounting of the blowing and calibrating mandrels and makes it possible to compensate for dimensional tolerances. If individual blowing and calibrating mandrels, or all of them, have to be replaced, or if individual blowing mandrels or all of them are changed in their position, for instance rotated about their longitudinal axis, then the mechanical clamping mounting of each individual blowing mandrel affected must be opened in order to perform the requisite change. After that, each mechanical clamping mounting is closed again and tightened to the requisite extent.